An impedance study of electron transport and electron transfer in composite polypyrrole plus polystyrenesulphonate films

At low potentials, the impedance of polypyrrole + polystyrenesulphonate changes from the simple transmission line response observed at high potentials to a more complex response including a high frequency semicircle in the complex plane representation. There is also a shift in the high frequency limiting real impedance as the electronic resistance R-e of the polymer becomes comparable with, and then greater than, the solution and film ionic resistances. The ionic conductivity of the polymer composite increases with decreasing potential, and becomes constant at its maximum value in the potential range where the changes in impedance behaviour occur. This greatly simplifies interpretation of the low potential impedance results, and allows unambiguous assignment of circuit elements. The charge transfer resistance R-ct, which causes the high frequency semicircles, is due to electron transfer at the polymer\electrode interface. Both R-ct and R-e decrease exponentially with increasing potential at 60 mV per decade. This observation validates the use, at low potentials, of theoretical models in which conducting polymers are treated as redox polymers despite their failure to follow the Nernst equation at higher potentials.